The present invention relates to a memory readout scheme for a three-layered thin-film EL display panel structure which manifests hysteresis memory effects when pletted with respect to applied voltage and light emission intensity, and more particularly to a readout scheme of a thin-film EL display panel having a matrix electrode pattern with an improved signal to noise (SN) ratio.
As disclosed and illustrated in U.S. Pat. No. 3,975,661 DRIVING METHOD FOR A THIN-FILM ELECTROLUMINESCENT ELEMENT OF A THREE-LAYER CONSTRUCTION assigned to the same assignee as this application, a thin-film EL display has a three-layered structure shown in a perspective view partly in cross section of FIG. 1, the disclosure of which is incorporated herein. A predetermined number of transparent electrode strips 2 are disposed on a glass support 1. Further, a layer 3 of dielectric material such as Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, TiO.sub.2 and Al.sub.2 O.sub.3, a layer 4 of electroluminescent material, for example, ZnS doped with Mn (yellowish orange light) and a second layer 31 of dielectric material such as Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, TiO.sub.2, Al.sub.2 O.sub.3 are disposed by a well known thin-film technique such as vacuum deposition and sputtering each having a thickness ranging from 500 to 10,000 A. This results in a double-isolation three-layered structure of the EL display panel. A different family of strip electrodes 5 is disposed in a direction normal to the direction of the transparent electrodes 2 to form an electrode matrix array together with the transparent electrodes. With such a three-layered thin-film EL display panel, if one of the first family 2 of the electrodes and one of the second family 5 of the electrodes are selected, a minute area, where the selected ones of the first and second family of electrodes cross each other, will emit light. This corresponds to a picture element of an image like a character, a symbol and a pattern being displayed.
The EL panel with a structure is more attractive than the prior art dispersed powder type EL panel from the standpoint of light intensity, working life and performance stability. The above described EL panel is more advantageous than the prior art in that it exhibits a hysteresis curve when plotted with respect to applied voltage and light emission intensity, as seen from FIG. 2(b). When first applying a pulse of the voltage amplitude V.sub.1 as shown in FIG. 2(a), the intensity of the EL panel is at the value B.sub.1 as shown in FIGS. 2(b) and 2(c). The sustained voltage V.sub.1 is correlated as V.sub.1 &gt;V.sub.th wherein V.sub.th is the threshold voltage level for light emission. Consecutive application of the sustained voltage V.sub.1 permits the intensity B.sub.1 to be held. If a write voltage V.sub.2 is then applied, the brightness will than go up to B.sub.3. Even if the sustained voltage V.sub.1 is reached again in a brief period, the intensity will settle at a value B.sub.2 higher than the previous one B.sub.1. Consecutive application of the sustained voltage V.sub.1 keeps the brightness B.sub.2 constant. When an erase voltage V.sub.3 is applied, this causes the intensity to drop. If the sustain voltage is reapplied, then the intensity will settle at B.sub.1. This time relationship is illustrated by using the same time references t.sub.1, t.sub.2 . . . t.sub.21 as in FIGS. 2(a) to 2(c). The hysteresis phenomenon may trace a different loop according to a selection of the amplitude, pulse width and frequency of the write voltage. In other words, a half tone display is also possible. Once the write voltage or the erase voltage has been applied in this way, the individual picture elements keep emitting light without losing their own tones, in response to the application of the sustain pulse. Though the respective levels of voltage depend mainly on compositions, thicknesses, manufacturing conditions and waveforms of applied voltages, the inventors' experiments revealed that V.sub.th =200 V, V.sub.1 =210 V, V.sub.2 =210 to 280 V and V.sub.3 =190 V, for example.
As noted earlier, the three-layered thin-film EL display panel is capable of reading out, electrically, the memory state thereof in addition to the functions of writing, erasing and sustaining by a proper selection of applied voltage, pulse width and pulse frequency.
The thin-film EL panel can be considered a capacitive element because of the fact that the electroluminescent layer sandwiched by the pair of dielectric layers, so that displacement current may flow through the EL panel when applying the sustain voltage. If the display element is latched in the light emitting state or write state, the displacement current will flow in response to the application of the sustain voltage plus current which is proportional to the brightness. This combined current is called polarization current. Actually because the background is held at some potential even in the erase state, a small amount of the displacement current will flow correspondingly. For example, while the EL element is supplied with the sustained voltage pulse as seen from FIG. 3(a), current flowing through the EL element will assume the waveform shown by the solid line I in FIG. 3(b) in the erase state and the waveform which is combined with the polarization current as depicted by the dotted line II in the write state.
The inventors' approach to determine the presence or absence of the polarization current involves a decision as to whether the displacement current exceeds a given amount taking a noise margin into account, thereby determining whether the EL element is in the write state or in the erase state. However, since this method is used to separate the displacement current from the polarization current at a predetermined level, a selection of the predetermined level needs careful consideration. The component of the polarization current over the predetermined level is equal to that minus the noise margin and the absolute value of this signal is extremely small. A signal-to-noise ratio is therefore not good.
It seems possible to remove undesirable current in the erase state by provision of an equivalent circuit consisting of analog elements such as capacitors and resistors which forms current similar to the displacement current or provision of a read only memory which stores the very waveform of that undersirable current in the erase state. Although these approaches are basically applicable to an EL unit element, the following problems will occur with respect to an EL matrix type element.
(1) In the case of matrix driving, the amplitude of the displacement current will vary greatly according to the number of the picture elements to be written and that to be erased. To this end it is rather difficult to form the equivalent circuit or the erase waveform.
(2) In order to efficiently cause light emission from the thin-film EL element, at least electrodes at the display side should be transparent. The transparent electrodes should be disposed at a much narrower interval in order to withstand a higher resistance as in the case where the display density is increased in a matrix display. This leads to a substantial difference in resistance between the display electrode closest to the lead region and that farthest from the same and therefore a substantial variation in the effective applied voltage results. Therefore, even though the same pulse is externally applied, the waveform of current will vary from point to point in the read out mode. This requires a predetermined number of equivalent circuits for storing the respective erase state current waveforms and thus renders a readout scheme complicated and large sized and expensive.
It is therefore an object of the present invention to provide an improved readout scheme for use in a matrix type EL display panel which overcomes the above described problems recognized by the inventors.
A matrix type thin-film EL display device embodying the present invention is provided at a portion of the display electrodes thereof in addition to a reference electrode. Current flowing through the reference electrode is utilized such that the erase state current or dark current cancels the displacement current. This makes it possible to pick up only the polarization current which is indicative of the internal state of the EL display device.